Method for producing a single-wall carbon nanotube

ABSTRACT

A method for producing a single-wall carbon nanotube, comprising contacting an organic dehydrated alcohol with a catalyst in a closed space in vacuum at a temperature of 600 to 900° C.

FIELD OF THE INVENTION

The present invention relates to a method for producing a single-wallcarbon nanotube, which is hereinafter also called “SWCNT” in some cases.

BACKGROUND

In order to produce a carbon nanotube which is hereinafter also called“CNT” in some cases, there have been conventionally known arc-discharge,laser ablation, and chemical vapor deposition.

Here, the arc-discharge is a method in which multiwall carbon nanotubesthat are hereinafter also called “MWCNT” in some cases, are produced onan anode by arc-discharging between carbon rods in an atmosphere such asargon at a pressure slightly lower than the atmospheric pressure. Inthis case, when arc-discharging is performed in the state that acatalyst such as Ni/Y is mixed into the carbon rods, SWCNTs can beformed on an inner side of a container. This arc-discharging method hasan advantage that CNTs having a relatively good quality can be producedwith fewer defects. To the contrary, however, this method has theproblems that (i) amorphous carbon is simultaneously produced, and (ii)it is costly and (iii) unsuitable for the mass production.

Meanwhile, the laser ablation is a method in which the CNTs are producedby irradiating a strong pulse beam such as YAG laser upon carbon intowhich catalyst such as Ni/Co is mixed in a high-temperature atmosphereof 900 to 1300° C. Although this method has the advantages that the CNTshaving a high purity can be obtained and the tube diameters can becontrolled by changing the conditions, the yield is low and it isdifficult to implement the method in an industrial scale.

Further, according to the chemical vapor deposition (CVD method), theCNTs is produced by bringing a carbon compound as a carbon source intocontact with fine particles of a catalytic metal at 500 to 1200° C., andboth of the MWCNTs and the SWCNTs can be produced. In addition, when acatalyst is arranged on a substrate, MWCNTs can be obtained, whileoriented vertically onto the surface of the substrate.

As the method for producing the SWCNTs by using the CVD method, a methodis known from a pamphlet of WO2003/068676, in which a carbon sourcecomposed of a compound having oxygen or a mixture of a compound havingoxygen and a compound having carbon is contacted with a catalyst at aheating temperature. However, a method has been sought, which canproduce SWCNTs and which can realize high growth rate, growthefficiency, vertical synthesis, etc.

SUMMARY OF THE INVENTION

The present invention is aimed at solving the above problems, and is toprovide a method for producing an SWCNT, which can realize high growthrate, growth efficiency, vertical synthesis, etc.

Having made keen examinations in view of the above problems, the presentinventors found out that those problems can be solved by the followingmeasures.

(1) A method for producing a single-wall carbon nanotube, comprisingcontacting an organic dehydrated alcohol with a catalyst in a closedspace in vacuum at a temperature of 600 to 900° C.

(2) The method for producing a single-wall carbon nanotube according to(1), wherein the pressure of the closed space at the time of contactingthe organic dehydrated alcohol is at most 10×10⁻² Pa.

(3) The method for producing a single-wall carbon nanotube according to(1) or (2), wherein the pressure at which the organic dehydrated alcoholis contacted with the catalyst is from 1 Pa to 100 kPa.

(4) The method for producing a single-wall carbon nanotube according toany one of (1) to (3), wherein the organic dehydrated alcohol mainlycomprises ethanol.

(5) The method for producing a single-wall carbon nanotube according toany one of (1) to (4), wherein the catalyst is at least one kindselected from Fe, Co, Ni, Mo, Pt, Pd, Rh, Ir, Y, La, Ce, Pr, Nd, Gd, Tb,Dy, Ho, Er and Lu and oxides thereof.

(6) The method for producing a single-wall carbon nanotube according toany one of (1) to (5), comprising washing the closed space beforecontacting the organic dehydrated alcohol with the catalyst.

(7) The method for producing a single-wall carbon nanotube according toany one of (1) to (6), wherein the catalyst is a particle and theprobability that a single-wall carbon nanotube grows from one particleis at least 50%.

(8) The method for producing a single-wall carbon nanotube according toany one of (1) to (7), wherein the single-wall carbon nanotube has alength of at least 50 nm.

(9) The method for producing a single-wall carbon nanotube according toany one of (1) to (8), wherein the single-wall carbon nanotube is formedon a substrate.

(10) The method for producing a single-wall carbon nanotube according toany one of (1) to (9), wherein the single-wall carbon nanotube can bevertically grown.

(11) The method for producing a single-wall carbon nanotube according toanyone of (1) to (10), wherein the organic dehydrated alcohol iscontracted with a catalyst at a temperature of 700 to 800° C.

(12) The method for producing a single-wall carbon nanotube according toany one of (1) to (10), wherein the pressure of the closed space at thetime of contacting the organic dehydrated alcohol is at most 10×10⁻⁴ Pa.

(13) The method for producing a single-wall carbon nanotube according toany one of (1) to (12), wherein the pressure at which the organicdehydrated alcohol is contacted with the catalyst is from 100 Pa to 40kPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a CVD apparatus used in Examples.

FIG. 2 is an AFM photograph of SWCNTs obtained in Example 1.

FIG. 3 is an enlarged photograph of a part of FIG. 2.

FIG. 4 is an enlarged photograph of a part of FIG. 3.

FIG. 5 is a diagram showing growth distribution proportions of theSWCNTs obtained in Example 1.

FIG. 6 shows a result of a Raman spectroscopic analysis (around 1200 to1800 cm⁻¹) of the SWCNT obtained in Example 1.

FIG. 7 shows a result of a Raman spectroscopic analysis (around 100 to400 cm⁻¹) of the SWCNT obtained in Example 1.

FIG. 8 shows an SEM photograph of SWCNTs obtained in Example 2.

FIG. 9 is an enlarged photograph of a part of FIG. 8.

FIG. 10 shows a sectional photograph of SWCNTs obtained in ComparativeExample 1.

FIG. 11 shows an enlarged photograph of a part of FIG. 10.

FIG. 12 shows a sectional photograph of SWCNTs obtained in ComparativeExample 2.

FIG. 13 shows an enlarged photograph of a part of FIG. 12.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In the following, the contents of the present invention will beexplained in detail.

Note that a range of “--- to ---” is used in the present specificationin a sense that figures before and after “to” are included as a lowerlimit and an upper limit of this range, respectively.

The method for producing the SWCNTs according to the present inventioncomprises contacting an organic dehydrated alcohol with a catalyst in aclosed space in vacuum at a temperature of 600 to 900° C.

That is, the closed space in the present invention is in vacuum when theorganic dehydrated alcohol is contacted with the catalyst. When thespace is in the vacuum state, remaining gases, materials, etc. caneasily removed, so that the SWCNTs having a high quality can beobtained.

Here, the “vacuum” means a state in which the pressure is reduced and nogas flows, and for example, it means the space obtainable by carryingout a vacuum exhaust of a closed space using a vacuum pump.

The degree of the reduced pressure of the vacuum space, that is, thepressure of the closed space when the organic dehydrated alcohol iscontacted with the catalyst is preferably at most 1×10⁻² Pa, morepreferably at most 1×10⁻⁴ Pa, and further preferably at most 1×10⁻⁵ Pa.

As such a closed space, a quartz tube can be recited. Further, thevacuum space can be attained by using one or more vacuum pumps,depending upon the degree of the vacuum.

The temperature inside the closed space in the present invention is setto a temperature at which the SWCNTs are formed from the organicdehydrated alcohol, and that temperature is preferably 600 to 900° C.,and more preferably 700 to 800° C. Such temperature conditions areordinarily set by raising the temperature in the state that the metalcatalyst is introduced into the closed space.

According to the present invention, the organic dehydrated alcohol iscontacted with the metal catalyst. The organic dehydrated alcohol in thepresent invention includes not only an organic dehydrated alcohol inwhich water is completely removed, but also an organic dehydratedalcohol containing water in small amount that it is ordinarily regardedas a dehydrated alcohol. For example, those which contain at most 0.005wt % of water are included in the organic dehydrated alcohol referred toin the present invention.

The kind, etc. of the organic dehydrated alcohol used in the presentinvention are not particularly specified, and two or more kinds of themixed alcohols suffice. As the organic dehydrated alcohol used in thepresent invention, mention may be made of, for example, methanol,ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, sec-butanol,tert-butanol, n-pentanol, iso-pentanol, n-amyl alcohol, iso-amylalcohol, n-hexanol, n-heptanol, n-octanol, n-nonylalcohol, n-decanol,etc. Among them, the organic dehydrated alcohol including at least onekind selected from methanol, ethanol and iso-propanol as a maincomponent is preferable, and one including ethanol as the main componentis more preferable. Here, the main component means a component havingthe maximum content proportion as calculated by weight, and preferablyamounting to at least 99.5 wt %.

The pressure at which the organic dehydrated alcohol is contacted withthe catalyst is preferably 1 Pa to 100 kPa, and more preferably 100 Pato 40 kPa, and further preferably 1 kPa to 4 kPa. When the organicdehydrated alcohol satisfying such conditions is contacted with thecatalyst, the method made possible a high growth rate and higher growthefficiency, even in a short-time growth.

In the present invention, the organic dehydrated alcohol is contactedwith the catalyst. The catalyst used in the present invention is notparticularly limited, and one or more kinds of the catalysts may be usedtogether. The metal catalyst is preferable. More specifically, the metalcatalyst is at least one kind of Fe, Co, Ni, Mo, Pt, Pd, Rh, Ir, Y, La,Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu and their oxides, and Co and Feare more preferable. As combinations of two or more kinds of thecatalysts, Fe/Co, Fe/Mo, Co/Mo, Fe/Ti, Fe/TiO₂, Fe/Al, Fe/Al₂O₃, Co/Ti,Co/TiO₂, Co/Al, and Co/Al₂O₃ are preferable.

The catalyst used in the present invention is preferably deposited onthe substrate. In order to deposit the catalyst on the substrate, aresist method, a dip coating method or the like can be employed.

As the resist method, any one of a negative type electron beam resist(for example, a method described in JAPP 43 (2004) 1356) and a positivetype electron beam resist (for example, a method described in J. AM.CHEM. SOC. 127 (2005) 11942) is preferable, and the positive typeelectron beam resist is more preferable. Further, when the positive typeelectron beam resist is employed, the catalyst is vapor deposited on aportion of the substrate where the resist is removed. At this time, itis preferable angularly to vapor and deposit the catalyst. This measureis preferably employed, because it can provide a catalyst patternsmaller than an actual pattern. The resist method is preferably for acase where a single CNT is grown on the substrate.

In the dip coating method, a catalyst layer is formed by dipping asubstrate into a solution containing a catalyst. When the dip coatingmethod is employed, the amount of the catalyst can be regulated byadjusting the concentration of the catalyst in the catalyst-containingsolution. The dip coating method is preferable in a case where finecatalyst particles are formed on the entire surface of the substrate.

Here, the substrate can be appropriately selected depending upon theuse, etc. of the SWCNTs produced, and is preferably silicon, SiO₂, andAl₂O₃. The surface of the substrate may be oxidized before the catalystis deposited. This treatment is preferably employed, because it islikely to suppress the formation of a silicide of the catalyst metal.

Further, any carrier may be provided between the substrate and thecatalyst.

In the producing method of the present invention, the inside of theclosed space is preferably washed before the organic dehydrated alcoholis contacted therewith. By employing this measure, remaining impuritiescan be removed, so that the SWCNTs can be more effectively grown.

The closed space is preferably cleaned by oxygen cleaning, ozonecleaning, plasma cleaning, vacuum thermal cleaning or the like. Theoxygen cleaning, the ozone cleaning, and the plasma cleaning arepreferable.

The pressure of the cleaning gas in the case of the cleaning withoxygen, the ozone cleaning, and the plasma cleaning is preferably 1 to100 kPa, and more preferably 1 to 3 kPa. On the other hand, the pressureof the cleaning gas in the case of the vacuum thermal cleaning ispreferably at most 1 Pa, and more preferably at most 6.6×10⁻⁴ Pa.

The temperature on cleaning is preferably 500 to 1000° C., and morepreferably 800 to 1000° C.

According to the producing method of the present invention, the SWCNTscan be produced in the state that the probability that the single-wallcarbon nanotube grows from one particle is high (the above probabilitybeing also called “growth rate” in some cases in this specification).The growth rate in the producing method of the present invention can bepreferably at least 50%, more preferably at least 60%, furtherpreferably at least 80%, and further more preferably 100%.

According to the producing method of the present invention, a SWCNT canbe grown long. Meanwhile, how long a SWCNT grows is called “growthefficiency” in some case in the present specification. According to theproducing method of the present invention, the SWCNT having a length ofpreferably at least 50 nm, more preferably at least 1000 nm, furtherpreferably at least 3000 nm can be obtained.

Furthermore, according to the present invention, the diameters of theSWCNTs can be 0.5 to 1.7 mm, and further can be 0.5 to 1.2 nm.

Moreover, according to the producing method of the present invention,the SWCNTs can be synthesized vertically to the substrate.

In addition, according to the present invention, variations in theconfigurations of the SWCNTs obtained can be reduced. For example, it ispossible that at least 60% (preferably at least 80%) of the SWCNTs canbe adjusted to fall in a diameter range of 0.5 to 1.5 nm.

The SWCNTs grown according to the method of the present invention can bepreferably used as a post-silicon material for field-effect transistorsand the like, probes for scanning type probe microscopes, field-emissiontype electron sources, etc.

Particularly, since the SWCNTs can be grown vertically to the carrier inthe form of the substrate, they can be expected to be applied to thefield-emission type electron source.

EXAMPLES

In the following, the present invention will be more concretelyexplained with examples. Materials, use amounts, proportions, processingcontents, processing procedures, etc. can be appropriately changed, solong as such changes do not deviate from the purpose of the presentinvention. Therefore, the scope of the present invention is not limitedto specific examples given below.

Example 1 Formation of Catalyst Patterns

Polymethylmethacrylate (PMMA) as an electron beam positive resist wascoated on that surface of a silicon substrate of which had beenthermally oxidized (hereinafter called also “thermally oxidized siliconsubstrate” in some cases) by spin coating, thereby obtaining a thin filmof about 50 nm. Next, rectangular patterns having sizes of 20 to 60 nmwere formed by electron beam lithography. Thereafter, Co was deposited,in the average thickness of 0.1 nm, on the resultant by vacuumdeposition, and Co patterns were formed by a lift-off technique withacetone.

Outline of an Apparatus

A chemical vapor deposition apparatus (CVD apparatus) constituted by aquartz tube, an electric furnace, a rotary pump and a turbo molecularpump as shown in FIG. 1 was used.

Cleaning of the Quartz Tube

Heat treatment was carried out, while an oxygen gas was being flown toclean the interior of the quartz tube and a quartz boat. The quartz tubewas vacuum evacuated down to 2 Pa (coarse sucking) by the rotary pump,and then while the oxygen gas was being flown at a flow rate of 0.5l/min., the tube was heated up to 800° C., and thermally treated at 800°C. for 10 minutes. At that time, the pressure was adjusted by switchinga valve to a needle valve so that the inner pressure of the quartz tubemight be about 2.7 kPa. After the cleaning, the oxygen gas was stopped,the quartz tube was cooled, while the tube was being evacuated to vacuumby the turbo molecular pump. The oxygen gas used was at a G1 grade(99.99995%). Cleaning can be also performed by the oxygen plasmacleaning, the ozone cleaning or the like.

Growing

A thermally oxidized silicon substrate on which the catalyst metal of Cowas formed in a discrete fashion was placed on the quartz boat, the boatwas placed in the heating furnace, the interior of the quartz tube wasvacuum evacuated down to 6.6×10⁻⁴ Pa by using the rotary pump and theturbo molecular pump, and the temperature was raised from roomtemperature up to 800° C. in about 15 minutes. After the temperaturereached the growing temperature, waiting was done for 10 minutes so thatthe temperature might be stabilized. At that time, the inner pressure ofthe quartz tube was 6.0×10⁻⁵ Pa (the pressure of the closed space at thetime of contracting the organic dehydrated alcohol). Subsequently,dehydrated ethanol for organic synthesis (water content: at most 50 ppm,manufactured by Wako Pure Chemical Industries, Ltd.) (pressure at thetime of introduction: 6.6×10⁻⁵ Pa) was introduced into the quartz tube,and SWCNs were grown in the closed quartz tube at the inner pressure of1.5 kPa (the pressure at which the organic dehydrated alcohol contactswith the catalyst) for 1 minutes. Thereafter, while the interior of thequartz tube was being evacuated to vacuum using the rotary pump and theturbo molecular pump, the furnace was cooled, and then the substrate wastaken outside, thereby obtaining SWCNTs. The pressure inside the quartztube immediately before taking the sample outside was 9.3×10⁻⁵ Pa.

A photograph of the SWCNTs obtained was taken by means of an atomicforce microscope (AFM). Results are shown in FIGS. 2 to 4. FIG. 3 is anenlarged photograph of a part of FIG. 2, and FIG. 4 is an enlargedphotograph of a part of FIG. 3.

Observation results confirmed that the yield of the SWCNTs was 60 to100%, and they grew at a very high efficiency. Further, it was confirmedthat the maximum length was at least 3 μm and the diameters were in arelatively narrow range of around 0.5 to 1.2 nm (FIG. 5). Further, FIG.6 and FIG. 7 show results in the Raman spectroscopic analysis(excitation wavelength: 488 nm) of a single SWCNT obtained. It was alsoconfirmed that the ratio (G/D) between the height of a peak near 1590cm⁻¹ and that of a peak near 1350 cm⁻¹ was 9.2 in FIG. 6 and that sincea peak was observed near 164 cm⁻¹ in FIG. 7, a SWCNT having a highquality was synthesized.

Example 2 Formation of a Catalyst Pattern

Catalyst patterns were formed according to a method described in Chem.Phys. Lett. 403 (2005) 320. Specifically, two kinds of solutions wereprepared, which had cobalt acetate tetrahydrate and molybdenum acetatedissolved in an ethanol solutions each in an amount of 0.01 wt %,respectively (cobalt acetate solution and molybdenum acetate solution).A thermally oxidized silicon substrate was dipped in the molybdenumacetate solution, then thermally treated at 400° C. in air for 5minutes, successively dipped in the cobalt acetate solution, andthereafter thermally treated at 400° C. in air for 5 minutes. Thereby,catalyst patterns in which a catalyst of Co/Mo is deposited on a surfaceof the substrate at a high density were obtained.

Next, after the quartz tube was cleaned in the same manner as in Example1, SWCNTs were grown in the same way as in Example 1.

A sectional photograph of the obtained SWCNTs was taken by a scanningelectron microscope (SEM). Results are shown in FIGS. 8 and 9.

It is also clear from FIG. 9 that the SWCNTs having the maximum lengthof 4 μm grew, while being oriented vertically to the substrate.

Comparative Example 1

SWCNTs were grown in the same manner as in Example 2, except that theorganic synthesis ethanol was replaced by a special-grade ethanol(manufactured by Kanto Chemical Co., Ltd.) having 0.4% of water at themaximum. The sectional photograph of the obtained SWCNTs taken by theSEM revealed that they did not grow vertically to the substrate, buthugging the substrate as shown in FIGS. 10 and 11, while no verticalgrowth was observed unlike the present invention.

Example 3

SWCNTs were grown in the same manner as in Example 2, except that theinterior of the quartz tube was not cleaned. Although the SWCNTsobtained were inferior to those in Example 2, the better SWCNTs wereobtained in Example 3 as compared with Comparative Examples 1 and 2.

Comparative Example 2

SWCNTs were grown in the same manner as in Example 2, except thatinstead of the vacuum evacuation of down to 6.6×10⁻⁴ Pa, the temperaturewas raised, while argon gas was filled inside the quartz tube at 0.3 SLMand a pressure of 40 kPa under controlling and that before growing, theinterior of the quartz tube was vacuum evacuated to 2 Pa through its oneend by the rotary pump. The sectional photograph of the obtained SWCNTstaken by the SEM revealed that they did grow not vertically to thesubstrate, but hugging the substrate as shown in FIGS. 12 and 13. Thus,it was recognized that no vertical growth was observed unlike thepresent invention.

Example 4

In Example 2, the interior of the quartz tube was evacuated to vacuum,and the temperature was raised to 800° C., while the pressure was keptat around 2 Pa, and then SWCNTs were grown at 2 Pa. A sectionalphotograph of the obtained SWCNTs taken by the SEM confirmed thatalthough the SWCNTs obtained were inferior to those in Example 2, thebetter SWCNTs were obtained in Example 4 as compared with ComparativeExamples 1 and 2.

The SWCNTs having excellent growth efficiency and growth rate could cometo be produced by employing the SWCNT producing method of the presentinvention. Further, the SWCNTs could come to be grown vertically to thesubstrate by employing the producing method of the present invention.

The present disclosure relates to the subject matter contained inJapanese Patent Application No. 160690/2006 filed on Jun. 9, 2006, whichis expressly incorporated herein by reference in its entirety.

All the publications referred to in the present specification are alsoexpressly incorporated herein by reference in their entirety.

The foregoing description of preferred embodiments of the invention hasbeen presented for purposes of illustration and description, and is notintended to be exhaustive or to limit the invention to the precise formdisclosed. The description was selected to best explain the principlesof the invention and their practical application to enable othersskilled in the art to best utilize the invention in various embodimentsand various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention not belimited by the specification, but be defined claims set forth below.

1. A method for producing a single-wall carbon nanotube, comprisingcontacting an organic dehydrated alcohol with a catalyst in a closedspace in vacuum at a temperature of 600 to 900° C.
 2. The method forproducing a single-wall carbon nanotube according to claim 1, whereinthe pressure of the closed space at the time of contacting the organicdehydrated alcohol is at most 10×10⁻² Pa.
 3. The method for producing asingle-wall carbon nanotube according to claim 1, wherein the pressureat which the organic dehydrated alcohol is contacted with the catalystis from 1 Pa to 100 kPa.
 4. The method for producing a single-wallcarbon nanotube according to claim 1, wherein the organic dehydratedalcohol mainly comprises ethanol.
 5. The method for producing asingle-wall carbon nanotube according to claim 1, wherein the catalystis at least one kind selected from Fe, Co, Ni, Mo, Pt, Pd, Rh, Ir, Y,La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu and oxides thereof.
 6. Themethod for producing a single-wall carbon nanotube according to claim 1,comprising washing the closed space before contacting the organicdehydrated alcohol with the catalyst.
 7. The method for producing asingle-wall carbon nanotube according to claim 1, wherein the catalystis a particle and the probability that a single-wall carbon nanotubegrows from one particle is at least 50%.
 8. The method for producing asingle-wall carbon nanotube according to claim 1, wherein thesingle-wall carbon nanotube has a length of at least 50 nm.
 9. Themethod for producing a single-wall carbon nanotube according to claim 1,wherein the single-wall carbon nanotube is formed on a substrate. 10.The method for producing a single-wall carbon nanotube according toclaim 1, wherein the single-wall carbon nanotube can be verticallygrown.
 11. The method for producing a single-wall carbon nanotubeaccording to claim 1, wherein the organic dehydrated alcohol iscontracted with a catalyst at a temperature of 700 to 800° C.
 12. Themethod for producing a single-wall carbon nanotube according to claim 1,wherein the pressure of the closed space at the time of contacting theorganic dehydrated alcohol is at most 10×10⁻⁴ Pa.
 13. The method forproducing a single-wall carbon nanotube according to claim 1, whereinthe pressure at which the organic dehydrated alcohol is contacted withthe catalyst is from 100 Pa to 40 kPa.
 14. The method for producing asingle-wall carbon nanotube according to claim 1, wherein the catalystis a particle and the probability that a single-wall carbon nanotubegrows from one particle is at least 60%.
 15. The method for producing asingle-wall carbon nanotube according to claim 1, wherein thesingle-wall carbon nanotube has a length of at least 1000 nm.